Solvent spinning salt solution processes

In this chapter we describe the use of simple ionic liquids based on ammonium salts with ammonia or other amino compounds, to dissolve cellulose and extrude filament. In the second section, the conversion of chitin and chitosan to filaments with other salt solutions is described. Both of these methods involve the direct dissolution of the polymer without derivatization. The solvent components include simple inorganic salts which can either be recovered and recycled back into the spinning process or used as fertilizer. 

In polyacrylonitrile appreciable electrostatic forces occur between the dipoles of adjacent nitrile groups on the same polymer molecule. This restricts the bond rotation and leads to a stiff, rodKke structme of the polymer chain. As a result, polyacrylonitrile has a very high crystalline melting point (317°C) and is soluble in only a few solvents, such as dimethylformamide and dimethylacetamide, and in concentrated aqueous solutions of inorganic salts, such as calcium thiocyanate, sodium perchlorate, and zinc chloride. Polyacrylonitrile cannot be melt processed because its decomposition temperature is close to the melting point. Fibers are therefore spun from solution by either wet or dry spinning (see Chapter 2). 

Aramid polymers have high melting points or melt with decomposition that makes fiber processing by melt spinning impractical [1] . Fibers are therefore spun from polymer solutions. These polymers not only do not melt but also are not easy to dissolve. Highly polar solvents, with or without the aid of inorganic salts such as lithium chloride or calcium chloride, or acids like concentrated sulfuric add have to be used [88]. 

A continuous dry spinning process has been developed where a solution of polymer, water, and salt is extruded into a gaseous medium [30]. The gaseous medium removes at least 25% weight percent of the solvent from the fiber. Then the fiber is quenched in an aqueous solution. After the fiber is conditioned, the fiber is eventually drawn. 

The processes used to prepare alumina based fibers by the solution route start with precursors which are either an aqueous solution of an aluminum salt or a solution of an organoaluminum compound in an organic solvent. The level of viscosity required for spinning is achieved by properly controlling the degree of hydrolysis/polycondensation of the precursor... 

Spinning of PPTA. Unlike MPDI, high molecular weight PPTA is not soluble in amide solvents, with or without the addition of inorganic salts. Formation of fibers from PPTA became possible when it was discovered that concentrated solutions of the polymer in 100% sulfuric acid had relatively low viscosity, could be spun at moderate temperatures, and that the PPTA did not degrade rapidly at those conditions (36). This discovery was the result of the study of nematic solutions of PBA and PPTA (22,36) and substantiated the theoretical predictions of Flory (39). A schematic of such a process is shown in Figure 6. 

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